This invention relates to dynamic focus control circuits for cathode ray display tubes, especially focussing at the extremities (corners, etc.) of a cathode ray tube screen. The invention is suitable for use in high definition video display systems.
In cathode ray tube display systems it is known that it is difficult to maintain a sharp focus of the cathode ray tube electron beam(s) over the total screen area, especially at the extremities, for instance corners. It is desirable to provide a focussing voltage which varies (dynamically) appropriately with beam position to maintain a sharp overall focus.
Systems for maintaining focus across the screen of a cathode ray tube display are known and utilize a dynamic focus voltage, the components of which vary in conjunction with horizontal and vertical sweep voltage in a manner similar to that used with dynamic pincushion and convergence correction systems. A difficulty met with in known focussing systems is in achieving the substantial dynamic focus component voltage changes required across the screen, especially as it relates to the horizontal sweep.